Purpose :
Animal models of myopia traditionally emphasize the changes in posterior parameters of the eye and associated elongation. Less attention has been paid to potential optical changes, so we evaluated 3D corneal and crystalline lens geometry in myopic and hyperopic guinea pig eyes following monocular lens treatment.

Methods :
Guinea pigs wore either a -6D (n=4) or a +5D (n=3) lens on one eye from 4-13 days of age. After 8 days of lens-wear, each eye was measured in anaesthetized animals using 3-D spectral OCT (300 Ascans x 50 Bscans in a 8x8 mm area; acquisition speed: 25.000 Ascans/s). OCT-images were obtained in two foci: (1) full anterior segment, (2) posterior lens and retina. The following parameters were extracted using custom-developed image processing and quantification routines: radii of curvature for the anterior and posterior corneal and crystalline lens (Rant/post C/L), asphericities (Qant/post C/L) and elevation maps (surface Zernike coefficients), corneal and lens thickness (CT, LT), anterior and vitreous chamber depths (ACD, VCD). Volume (VOL), Diameter (DIA) and equatorial plane position (EPP) of the lens were estimated from full reconstructions from partial images. Refractive error was measured in cyclopleged eyes using streak retinoscopy.

Results :
The mean difference between the eyes (diff) in refractive error was -6.03D after minus lens wear and +1.53D after positive lens wear, and was accompanied by longer or shorter VCDs respectively (diff: +230 Vs. -56 µm). Bidirectional changes also occurred in the corneal radii, becoming steeper after negative and flatter after positive lens-wear (diff in myopes: -130 µm and -51µm (ant), diff in hyperopes: +106 µm and +19 µm (post)). The crystalline lens was thicker and had a greater volume after negative lens wear (diff of 30 µm and 1.2 mm3 respectively). The magnitude of astigmatism was x3.5 (ant cornea) and x2.6 (posterior lens) higher in the myopic eyes in comparison with the hyperopic eyes; higher-order RMS surface elevation was largest in the posterior lens of the myopic eyes (x2.2; vertical coma was the dominant coefficient).

Conclusions :
Sign dependent changes were observed in the corneal radius and the vitreous chamber depth, and significant changes occurred in the crystalline lens. Therefore, exposure to retinal defocus systematically changes the optical properties of the developing eye.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.